In the arts, inkjet printers have a paper path that moves the paper in one axis of motion and a carriage that moves back and forth (reciprocates) over the paper while the carriage's inkjet heads are ejecting ink. Popular ink-jet printing systems have four (4) printheads aligned horizontally and made to scan side-to-side in order to print a single swath of an image. A swath is a strip of printed image that is equal to the height of the print heads. This design helps keep the platen under the paper as narrow as possible. One disadvantage of putting all the heads in-line is that, although the same primary colors are used on each pass of the carriage, the order in which the colors are laid down determines, to some extent, the resulting composite colors produced. After each swath is printed, the paper is advanced vertically prior to printing the next swath. After each printing pass the media is moved one head height (or a fraction thereof) and the carriage again moves across the paper.
In a reciprocating carriage print head where the print heads are aligned horizontally in the scan direction, it is sometimes desirable to print in a single-pass, bi-directional mode as increased productivity is often obtained when printing in this mode because the printhead assembly prints all image pixels in a given swath by scanning from left-to-right in the swath, having the paper advance vertically, and then printing all image pixels in the next swath by scanning from right-to-left. After the paper advance, the process starts over with a left-to-right scan.
For the most part, color differences are due to the order in which the ink is ejected on the paper. In one swath the inks are laid down in a left-to-right order, whereas in the following swatch, the inks are laid down in a right-to-left order. When printing left-to-right, the inks are usually ejected in the order of: yellow, magenta, cyan, black. When printing right-to-left, the inks are usually ejected in the order: black, cyan, magenta, yellow. A result is that a red color printed in one swath may not have the same appearance in a successive swath because the red color produced by printing yellow first followed by magenta on top is not necessarily the same red produced when printing magenta with yellow on top. When laying light cyan ink on top of dark cyan, the color will be different than if the dark cyan ink were laid on top of the light cyan. This is due to differences in the individual inks absorption and scattering properties, the total area coverage of each of the inks, and the halftone algorithm. In general, this means that certain colors such as reds, blues, browns, flesh tones, etc. that can be obtained with one direction of printing may not necessarily be able to be obtained with the reverse direction of printing. These differences are often referred to as color banding and appear as alternating stripes equal in height to the height of the head.
To get a typical printer's output to match a standard other than the standard that the ink-set was designed to match (assuming it was designed to match a standard), a method called color profiling is typically used. Color profiling is an attempt to characterize the printer's colorimetric reproduction characteristics given a specific set of inks, media, and environmental conditions and use this information along with color correction data that attempts to get the printer's output to match a standard.
Attempts at reducing color banding have relied on employing different color calibration tables for left-to-right and right-to-left swaths. In this approach, two color calibration tables are generated. The first table is produced by printing a calibration target in a single-pass, unidirectional mode wherein each swath is printed from left-to-right without printing right-to-left. The second table is produced by printing a calibration target in a single-pass, unidirectional mode wherein each swath is printed from right-to-left without printing left-to-right. After both calibrations are complete, two tables are generated and used to produce prints in a single-pass, bi-directional mode. This is done by processing image data contained within swaths printed by scanning the printhead assembly from left-to-right by using the left-to-right color look-up table, and processing image data contained within swaths printed by scanning the printhead assembly from right-to-left by using the right-to-left color look-up table.
Although, in principle, this should be successful at reducing color banding in single-pass, bi-directional printing, the actual results still contain varying degrees of color banding. This is because the color gamut obtainable by printing in one direction is not necessarily the same as the color gamut obtainable by printing in the reverse direction. For example, assume it is desirable to produce a solid red at the very edge of the red gamut. This would be done by printing 100% yellow and 100% magenta on the paper. As discussed, the magenta will be on top of the yellow in one direction and the yellow will be on top of the magenta in the other. Since the maximum amount of ink per color per pass is 100%, the resulting red will not look the same on successive swaths as the size and shape of individual color regions in the left-to-right gamut may be different relative to the regions in the right-to-left gamut, and visa versa. If the only concern was gamut mapping independently for both directions, the results will be predictably poor. There are several reasons for this, relating to gamut size, color resolution, and the accuracy of the interpolation method used to determine intermediate values. If the gamut is too small or the color resolution too high, or if the interpolation method is inaccurate, it is unlikely that the color correction software will come up with identical combinations of primary colors for both directions.
Thus, what is needed in this art is a method, which provides improvement in image quality in print systems that have noticeable (uncorrected) color shifts between the two directions.